This invention relates to a yoke mechanism for a printing apparatus using an electrophotographic method.
In order to reduce the cost of a yoke mechanism and reduce the mechanism to compact size, a double face tractor in which both sides of the tractor are used, as illustrated in FIGS. 1 and 2, is in general use. An electrophotographic printing apparatus 1 comprises a corona charger 3 for applying a charge to a photoconductor 2, an optical portion 4 for writing information on the photoconductor 2, a developing device 5, a yoke mechanism 7 for feeding printing paper 6 at a uniform speed and transferring an image on the photoconductor 2 onto the printing paper 6, a fixing device 8 for heat fixing the transferred image, and a stacker 9 for folding the printing paper after fixing.
A double face type (using both sides) tractor 10 for feeding the printing paper 6, a retractor 11 for bringing the paper into contact with and separating it from the photoconductor 2 at time of transfer, and a transfer charger 12 are disposed in the interior of the yoke mechanism 7. Paper presses 10a, 10b are freely pivotally supported on both sides of the tractor 10, respectively. One end of the yoke mechanism 7 is pivotally supported on a frame 13 by a hinge 7a in such a manner that when the yoke mechanism 7 is opened, the rear thereof is adapted to abut a stopper 13a projecting backwardly from the frame 13 to be stopped.
This position of the yoke mechanism is called the retract position. When the yoke mechanism 7 is closed, the forward end thereof is adapted to abut a pin 14 mounted on the front of the frame 13 to stop the yoke mechanism. This position of the yoke mechanism is called the transfer position, where the retractor 11 approaches the photoconductor surface 2 in such a manner as to perform a transfer step.
Conventionally, when loading such a yoke mechanism with printing paper, as shown in FIG. 2, the paper presses 10a, 10b on both sides of the tractor 10 are opened to place the paper 6 therein with the yoke mechanism 7 fixed perpendicularly. This method is very complicated to perform because the paper 6 is inserted from laterally of the tractor 10.
In another method, the paper 6 is placed on the side of the paper press 10a of the tractor 10 at the retracted position, and is then placed on the side of the press 10b of the tractor 10 with the yoke mechanism 7 closed. This is easy because the paper is placed in a substantially horizontal state. But when the yoke mechanism 7 is closed while the paper is fixed on only one side of the tractor 10, the slack paper 6 can directly contact the soft surface of the photoconductor 2, which sometimes causes the photoconductor 2 to be damaged. Accordingly, the expensive photoconductor 2 becomes unusable.